The efficient utilization of phosphorus (P) is crucial for maximizing crop yields, particularly in acidic soils where P availability is often limited. Soil acidification, frequently exacerbated by the long-term application of inorganic nitrogen fertilizers, significantly impacts P availability and overall crop productivity. In many parts of the world, including China, the extensive use of urea and other nitrogen fertilizers has led to widespread soil acidification, negatively affecting soil biochemical properties and plant growth. Acidification alters the balance of acidic cations (H⁺ and Al³⁺) and base cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺), impacting nutrient availability. The increased concentration of H⁺ ions displaces essential base cations, leading to nutrient depletion and increased solubility of toxic metals like aluminum (Al), iron (Fe), and manganese (Mn). Phosphorus availability is particularly sensitive to soil pH; in acidic conditions, P fixation with Al and Fe reduces plant P uptake, lowering phosphorus use efficiency (PUE). While organic amendments can improve soil pH and PUE, liming (application of CaO) is considered a highly effective method for mitigating soil acidification and enhancing P availability. However, the optimal combination of liming and fertilization strategies for enhancing both crop yield and PUE in acidic soils requires further investigation. This study aimed to investigate the long-term effects of liming and fertilization on soil chemical properties, PUE, and crop yields under a wheat-maize rotation system in acidic soil.

Numerous studies have highlighted the detrimental effects of soil acidification on crop productivity and nutrient use efficiency. Long-term inorganic nitrogen fertilization has been linked to significant soil acidification globally, impacting nutrient availability and plant growth. The use of various organic and inorganic amendments, including liming, has been explored to counteract soil acidification and enhance phosphorus use efficiency. Previous research indicates that combining organic amendments like wheat straw or pig manure with inorganic fertilizers can increase phosphatase activities and PUE. Lime application has been shown to increase P availability in acidic soils by reducing the levels of P-fixing cations, like Al and Fe. However, the relationship between soil pH and P availability is complex and can vary depending on soil conditions and P input levels. High P inputs in field experiments can lead to the precipitation of Al phosphate, further complicating P dynamics. The understanding of the interactions between liming, fertilization, and soil chemical properties is critical for developing effective strategies to optimize crop production in acidic soils. Studies conducted in Chinese croplands, where soil acidification is a major problem, have demonstrated the need for interventions to improve nutrient use efficiency and crop yields.

A long-term field trial was conducted from 1990 at the National observation and research station of farmland ecosystem, Qiyang county, China. The experimental site had a subtropical monsoon climate with a mean annual temperature of 17.8 °C and mean annual rainfall of 1290 mm. The soil was classified as Eutric Cambisol (WRB), Inceptisol (USDA), and red soil (Chinese classification system), with a light loam texture. Seven treatments were employed in a split-plot design with two replicates: CK (control, no fertilization), NP (N and P), NPK (N, P, and K), NPKS (NPK + straw), NPCa (NP + lime), NPKCa (NPK + lime), and NPKSCa (NPKS + lime). Fertilizers were applied annually before sowing, with urea at 150 kg N ha⁻¹, calcium superphosphate at 120 kg P₂O₅ ha⁻¹, and potassium chloride at 120 kg K₂O ha⁻¹. Lime (CaO) was applied at 2550 kg ha⁻¹ in 2010 and 1500 kg ha⁻¹ in 2014. Wheat (Xiangmai cultivar) and maize (Yedan-13 cultivar) were sown each year. Soil samples (0–20 cm) were collected annually after maize harvest for analysis of pH, organic carbon (SOC), total N, available N, total P, available P, total K, available K, exchangeable Ca²⁺, Mg²⁺, and Al³⁺. Phosphorus concentration in grain and straw was determined using the vanadomolybdate yellow method. PUE (kg kg⁻¹) was calculated using the equation: PUE = (YF − Y0)/F, where YF is the annual crop yield under fertilization, Y0 is the control yield, and F is the annual P input. Data were analyzed using one-way and two-way ANOVA, Tukey’s HSD test, linear regression, and boosted regression tree (BRT) analysis.

Long-term fertilization without lime significantly decreased soil pH and crop yields compared to treatments with lime application (p≤0.05). Average soil pH values across the years decreased significantly under the treatments without lime (NP, NPK, NPKS) compared to the control, while it increased under the treatments with lime (NPCa, NPKCa, NPKSCa) compared to control treatment. Compared to the control (CK), wheat grain yield increased by 138–688% and maize yield increased by 687–5077% across the various treatments. Lime application significantly increased exchangeable Ca²⁺ and Mg²⁺ while decreasing exchangeable Al³⁺. Compared to the NP treatment, PUE increased by 212–807% under treatments with lime and additional nutrients. Soil pH showed a significant negative relationship with exchangeable Al³⁺ and total N, and a significant positive relationship with exchangeable Ca²⁺, PUE, and annual crop yield. PUE was highly negatively correlated with exchangeable Al³⁺. BRT analysis revealed that exchangeable Ca²⁺, pH, exchangeable Al³⁺, and available N were the most influential factors on crop yield.

The findings confirm the detrimental impact of long-term inorganic fertilization without liming on soil acidification and crop productivity. The significant increase in crop yield and PUE in lime-amended treatments highlights the effectiveness of liming in mitigating soil acidity. The positive correlation between soil pH and PUE, along with the negative correlation between PUE and exchangeable Al³⁺, underscores the crucial role of soil pH in regulating P availability. The BRT analysis further confirms the major influence of exchangeable Ca²⁺, soil pH, exchangeable Al³⁺, and available N on crop yield, supporting the crucial role of maintaining a favorable cation balance in acidic soils for optimizing crop production. The increased P uptake in treatments with lime suggests a decreased fixation of P by Al and Fe oxides, which allows for greater P availability for plants. The differences in available P among the different treatments suggest that the optimum balance between lime and P fertilizer application requires further optimization.

This study demonstrated that long-term fertilization without liming resulted in significant soil acidification, reducing crop yields and PUE. Liming effectively mitigated soil acidification, increased PUE, and substantially enhanced crop yields. The combined application of fertilizers, lime, and crop straw represents a promising strategy for achieving high crop yields and PUE in acidic soil under wheat-maize rotation systems. Future research should focus on optimizing lime and fertilizer rates for different soil conditions and crop types to further improve efficiency and sustainability.

The study was conducted at a single location, limiting the generalizability of the findings to other regions with different soil types or climates. The long-term nature of the experiment inherently limits the ability to account for year-to-year variations in rainfall or temperature, potentially influencing the results. Although efforts were made to minimize contamination between plots, some level of cross-contamination may have occurred, potentially affecting the precision of the results.